1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and particularly, to an LCD device capable of efficiently radiating light emitted from an optical source by increasing surface areas of a lower cover, an upper cover, and a guide panel.
2. Background of the Invention
Recently, various portable electric devices, such as mobile phones, personal digital assistant (PDA), and note book computers have been developed, because of their small size, light weight, and power-efficient operations. Accordingly, flat panel display devices, such as liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), and vacuum fluorescent displays (VFDs), have been developed. Of these flat panel display devices, the LCDs are currently massively produced because of their simple driving scheme and superior image quality.
The LCD device is a transmissive type display device, and displays a desired image on a screen by controlling an amount of light passing through a liquid crystal layer by a refraction anisotropy of a liquid crystal molecule. Accordingly, the LCD device is provided with a backlight unit, an optical source passing through a liquid crystal layer for an image display. The backlight unit is generally divided into an edge type backlight unit that a lamp is installed on a side surface of a liquid crystal panel thus to provide light to a liquid crystal layer, and a direct type backlight unit that a lamp is installed below a liquid crystal panel thus to directly provide light to a liquid crystal layer.
According to the edge type backlight unit, a lamp is installed on a side surface of a liquid crystal panel thus to provide light to a liquid crystal layer through a reflector and a light guide plate. Accordingly, the edge type backlight unit has a thin thickness thereby to be mainly applied to a notebook, etc. However, the edge type backlight unit has a difficulty in being applied to an LCD panel having a large area since a lamp is installed on a side surface of an LCD panel. Furthermore, since light is supplied through the light guide plate, it is difficult to obtain high brightness. Accordingly, the edge type backlight unit is not suitable for an LCD panel for an LCD TV having a large area, one of the most spotlighted LCD panel.
According to the direct type backlight unit, light emitted from a lamp is directly supplied to a liquid crystal layer. Accordingly, the direct type backlight unit can be applied to a liquid crystal panel of a large area, and a high brightness can be implemented. Therefore, the direct type backlight unit is mainly used to fabricate a liquid crystal panel for an LCD TV.
As an optical source of the backlight unit, a Light Emitting Device (LED) which spontaneously emits light, rather than fluorescent lamps, is being used. The LED is an optical source which spontaneously emits monochromatic light such as R, G and B. Accordingly, when being applied to the backlight unit, the LED implements an excellent color reproduction rate, and reduces driving power.
FIG. 1 is a view showing a structure of an LCD device having a backlight unit provided with an LED in accordance with the conventional art.
As shown in FIG. 1, the LCD device comprises an LCD panel 10 composed of a first substrate 1, a second substrate 3, and an LC layer (not shown) disposed therebetween, for implementing an image as a signal is applied thereto from outside; a light emitting device (LED) 52 disposed at one lower side surface or two lower side surfaces of the LCD panel 10, for emitting light; a light guide plate 20 disposed below the LCD panel 10, for guiding light emitted from the LED 52 to the LCD panel 10; an optical sheet 30 disposed between the LCD panel 10 and the light guide plate 20, for enhancing optical efficiency by diffusing and collecting the light supplied to the LCD panel 10 under guide of the light guide plate 20; a reflector 27 disposed below the light guide plate 20, for reflecting incident light to the LCD panel 10; a guide panel 25 for supporting the light guide plate 20 and the LCD panel 10; a lower cover 40 disposed below the guide panel 25, for assembling the LCD panel 10, the light guide plate 20, the optical sheet 30, and the guide panel 25; and an upper cover 42 for supporting the LCD panel 10, the light guide plate 20, the optical sheet 30, and the guide panel 25 by being coupled to the lower cover 40.
Although not shown, a plurality of pixels are provided at the first substrate 1 of the LCD panel 10, and a pixel electrode and a thin film transistor (TFT) are provided at each of the pixels. A common electrode is formed at the second substrate 3. When a signal is applied to the LCD panel 10 from outside through the TFT, an electric field is formed at an LC layer to control alignment of LC molecules. Accordingly, transmittance of light which passes through the LC layer is controlled, thereby implementing an image. The first substrate 1 and the second substrate 3 are respectively provided with a polarizer (not shown) attached thereto. The polarizer serves to control a polarization direction of light made to be incident to the LC layer, and light emitted from the LC layer.
However, the conventional LCD device has the following problems. In the case of using the LED rather than fluorescent lamps such as Cold Cathode Fluorescent Lamps (CCFL) as an optical source, heat is generated from the LED 52. The heat is radiated through the lower cover 40. However, most of the heat is not radiated, but remains at the backlight unit. This may cause the backlight unit to have temperature increment to about 80˜90. This temperature of 80˜90 is higher than that of a backlight unit having CCFLs by about 40˜50. This temperature increment of the backlight unit causes deformation of the optical sheet, damage of a junction of the LED, decrement of optical efficiency of the LED, degradation of liquid crystal on the panel, thereby lowering reliability of main components of the LCD device.